Epoxy resins are one of the most important classes of thermosetting polymers with greater than (>) about 50 percent (%) being used for protective coatings such as weatherable coating used in maintenance and protective coating (M&PC) applications. Known epoxy resins useful in coating applications include for example bisphenol A diglycidyl ether (BADGE) which are popular in the industry (e.g., >75% of resin sales volume) because BADGE is readily available in the industry and because BADGE exhibits a good balance of properties.
However, BADGE based articles exhibit poor ultraviolet (UV) light resistance; and the poor UV light resistance property of BADGE based articles creates difficulties with yellowing and chalking in exterior applications. The poor ultraviolet (UV) light resistance of BADGE based articles is due to aromatic ether groups present in the chemical structure of BADGE because the aromatic ether groups present in the chemical structure of BADGE tend to absorb UV radiation leading to photooxidative degradation. For this reason, epoxy resin coatings made from BADGE are often overcoated with a durable top coat made from for example polyurethane, an alkyd, or an acrylic composition in order to impart protection to the epoxy under coating.
Approximately 80% of the cost of a protective coating system comes from labor to prepare the surface and apply the coating. If an epoxy coating could be made with weatherable properties and useful in applications such as anticorrosion coating systems without the requirement for a top coat, such epoxy coating could provide significant systems savings in terms of costs of materials and labor efficiencies.
Some non-aromatic epoxy resin compounds are inherently UV resistant because the epoxy resin compounds lack aromatic ether linkages. For example, 1,4-cyclohexanedimethanol (CHDM) epoxy resin and Unoxol™ (an epoxy resin which is a mixture of 1,3 and 1,4 cis and trans cyclohexanedimethanol epoxy resin), are both aliphatic epoxy resins; and such resins lack aromatic ether linkages, but instead contain aliphatic linkages. Thus, such aliphatic epoxy resin compounds are inherently more UV resistant than aromatic epoxy resins.
However, aliphatic epoxides do not react effectively with conventional nucleophilic epoxy curing agents, such as amines, at ambient temperature (e.g. at about 25° C.). The lower reactivity at ambient temperature arises because (i) the aliphatic epoxides are less susceptible to nucleophilic attack due to the lower electronegativity of the cycloaliphatic ring relative to the aromatic ring in standard epoxies, and (ii) conventional amine curing agents lack compatibility with cycloaliphatic epoxy resins.
Furthermore, curing agents used with cycloaliphatic epoxy resins often require additional accelerators, such as DMP-30 or salicylic acid to achieve ambient cure. The accelerators often negatively affect coating performance by introducing aromatic groups, particularly aromatic groups with a heteroatom directly bound to the aromatic ring, into the coating formulation. Such curing agents pose a barrier which has historically prevented cycloaliphatic epoxy resins from use in ambient temperature cure M&PC applications.
For example, U.S. Pat. No. 5,310,770 discloses water-reducible curing agents for epoxy resins made from the diglycidyl ether of cyclohexane dimethanol and a diamine selected from the group consisting of meta-xylylene diamine, isophorone diamine, diaminocyclohexane and 1,3-bisaminomethyl cyclohexane. These curing agents are soluble in water and are useful for curing epoxy resin compositions. U.S. Pat. No. 5,310,770 discloses an aqueous solution of a hardener made at an epoxy to amine stoichiometric ratio of 1:2. U.S. Pat. No. 5,310,770 does not disclose coating formulations nor cured coating compositions. In addition, U.S. Pat. No. 5,310,770 does not teach the use of amine adducts to improve the compatibility and the reactivity of such adducts with cycloaliphatic epoxies. Also, no curing accelerators nor stabilizers are mentioned in U.S. Pat. No. 5,310,770. Nothing in U.S. Pat. No. 5,310,770 teaches the use of adducts based on weatherable curing agents and cycloaliphatic epoxy for weatherable coating applications.
WO2009142898(A1) discloses adducts from UNOXOL epoxy resin. The homogeneity of adduct with epoxy resin not a requirement in WO2009142898(A1). In addition, ambient temperature cure is not demonstrated in WO2009142898(A1).
Other epoxy-amine adducts are known in the art including WO2009142898(A1); U.S. Pat. Nos. 4,310,695, 3,629,181, 5,246,984, 3,496,130, 3,639,344 and 4,113,684. None of the above reference teach adducts suitable for curing epoxy resins, wherein the adduct is prepared by reacting an epoxy resin with an amine and the adduct is used in weatherable coating applications.
In addition, epoxy resins prepared by reacting aliphatic and cycloaliphatic diols with Lewis acids typically results in an epoxy resin containing a significant amount of chlorine due to oligomerization of epichlorohydrin onto the alcohol functionality. This bound chlorine provides unwanted sites for reaction with amines during adduct preparation and results in the release of chlorides into a composition. These high chloride containing adducts have an undesirable high viscosity and low reactivity. For example U.S. Pat. No. 4,310,695 teaches removal of chlorine from an adduct by converting the adduct to hydroxyl functionality by hydrolysis in order to avoid problems when chlorine is present in the adduct. When high chloride containing adducts are subsequently cured with epoxy to yield coatings, these high chloride containing adducts provide low coating performance properties such as gloss, water resistance, and corrosion resistance.